Dynamic regulation of follicle-stimulating hormone-beta messenger ribonucleic acid levels by activin and gonadotropin-releasing hormone in perifused rat pituitary cells.

ABSTRACT

Maintenance of FSH biosynthesis requires ongoing exposure to pulsatile GnRH. Recent data demonstrate that activin also stimulates FSH biosynthesis. We used a perifused pituitary system to examine regulation of FSH beta mRNA levels by pulsatile GnRH and activin. Hourly pulses of 10 nM GnRH increased FSH beta mRNA levels by 3-fold. In the same experiment, continuous infusion of 50 ng/ml activin elicited a 50-fold increase in FSH beta mRNA. This magnitude of response to activin in perifusion was unexpected, as only a 2.7-fold increase in FSH beta mRNA was observed when activin was administered to pituitary cells that were cultured in dishes. Since perifusion columns, unlike culture dishes, are exposed to a continuous supply of fresh medium, we examined the possibility that endogenous factors produced by pituitary cells cultured in dishes were stimulating the cells in a paracrine fashion, thereby precluding the full response to exogenously added activin. The kinetics of FSH beta mRNA expression were examined immediately after pituitary dispersion and at different times after culturing the cells in plates. FSH beta mRNA levels fell rapidly after dispersion to 8% of initial levels and remained low over 8 h. Thereafter, FSH beta mRNA levels increased slowly and exceeded initial levels by the second day of culture. In a parallel set of experiments, when medium conditioned by exposure to plated cells was applied to the perifusion system, FSH beta mRNA levels were selectively stimulated (6-fold). These data suggest the removal during dispersion and subsequent accumulation in culture of pituitary-derived factors that are important for the maintenance of FSH beta mRNA levels. We conclude that activin plays a greater role in the regulation of FSH beta mRNA levels than was suggested by previous experiments employing static culture systems in which autocrine or paracrine stimulation may have obscured the effects of exogenously added activin.